1. Field of the Invention
The present invention relates to a composite gear blank comprising a rigid centerpiece and an outer part made of a plastic material. The invention also relates to the use of such a composite gear blank and to methods for manufacturing the same.
2. Description of the Related Art
Electronic power steering (EPS) systems eliminate the traditional hydraulic systems' power steering pump, hoses, hydraulic fluid, and drive belt and pulley on the engine. EPS significantly reduces the amount of energy drawn from the engine, thus improving fuel economy, acceleration, and dependability. EPS offers the feel and handling of a traditional hydraulic-powered system, without the loss of power that results from a hydraulic pump running off belts and pulleys driven by the motor.
A typical EPS system has an electric motor, sensors and a worm-gear assembly. The requirements are:                resistance, during steering maneuvers, to both the maximum input torque and resultant axial force of the worm gear;        reduction of vibration and noise;        low friction and precision tolerances for all gear surfaces;        ability to be press-fit to a steel shaft;        durability and fatigue resistance for the life of the vehicle.        
For reasons of noise reduction, lower coefficient of friction and wear reduction, the toothed peripheral portion of the gear wheel is preferably made of a synthetic, thermoplastic material. On the other hand, the central hub portion is preferably made of metal, so as to allow firm connection of a metallic shaft. As a consequence, this requires production of a composite piece having a plastic outer part firmly attached to a metallic inner part.
To achieve the above task, various approaches have been described in the prior art, see e.g. WO 2005/104692 A2 and references cited therein.
U.S. Pat. No. 6,638,390 B1 discloses a process for manufacturing polymer/metal disks wherein a bonding agent is applied to a metallic hub, followed by pressing a polymeric ring over the metallic hub to form a ring/hub assembly. The assembly thus formed is then heated to cause bonding between the polymeric ring and the metallic hub. A similar process has been disclosed in JP 2003/118 006 A.
EP 1 609 694 A1 relates to an electric power steering device and resin gear used for the same. According to the method described in this document, the plastic ring is made by a separate casting step. Subsequently the ring is heated and press-fitted onto a slightly larger metal core.
EP 1 690 664 A1 describes a method of resin coating an inert member by means of injection molding.
DE 101 27 224 A1 discloses a toothed wheel, especially for a worm gear mechanism, consisting of a metal disc with a central opening for a shaft, a ring shaped plastic element on the circumference of the disc and a toothed crown. In one embodiment, it is contemplated to form the ring shaped plastic element by casting (in German: “formschlüssiges Angiessen”) around a massive metallic hub. In another embodiment, it is contemplated to form the plastic element by injection molding (in German: “Spritzgiessen”). However, DE 101 27 224 A1 does not give any detailed information on how to carry out the casting or injection molding processes nor does it show any comparison of the results obtained by the two methods.
U.S. Pat. No. 6,432,343 B1 describes a process of manufacturing rotationally symmetric articles such as rope pulleys, rope sheaves, running wheels and the like, which optionally can have a metallic hub for reinforcement. The manufacturing process involves a centrifugal casting method according to which a first ε-caprolactam melt is poured into a rotating mold. When the reactive melt starts to polymerize, at which time it is also being pushed radially outwards against the peripheral mold surface due to the mold rotation, a second ε-caprolactam melt is poured into the mold for formation of the hub area. This second melt is selected so as to have increased strength properties in comparison to the first melt. As an option, it is contemplated to place a metallic part at the center of the mold so as to form a hub of the rotationally symmetric article.
However, it has been found that the strength of the plastic to metal connection of the composite gear blanks produced according to the various prior art methods does not meet some of the stringent requirements for EPS systems in larger engine cars and/or for EPS-systems located “under the hood”, i.e. close to the engine where the operating temperature is typically around 120° C.
Accordingly, it would be highly desirable to have a composite gear blank with a substantially improved adhesion strength between the plastic outer part and the rigid centerpiece, even at elevated temperatures.
Moreover, it would be desirable to have a manufacturing method allowing for an efficient and reliable production of such improved composite gear blanks.